Lesson 4 : Newton's Third Law
of Motion

Lesson 2: Force and Its
Representation

Types of Forces

A force is a push or pull acting upon an object as a
result of its interaction with another object. There are a
variety of types of forces. Previously
in this lesson, a variety of force types were placed
into two broad category headings on the basis of whether the
force resulted from the contact or non-contact of the two
interacting objects.

Contact
Forces

Action-at-a-Distance
Forces

Frictional Force

Gravitational Force

Tension Force

Electrical Force

Normal Force

Magnetic Force

Air Resistance Force

Applied Force

Spring Force

These
types of individual forces will now be discussed in more
detail. To read about each force listed above, continue
scrolling through this page. Or to read about an individual
force, click on its name from the list below.

Type
of Force

(and Symbol)

Description of
Force

Applied Force

Fapp

An applied force is a force
which is applied to an object by a person or
another object. If a person is pushing a desk
across the room, then there is an applied force
acting upon the object. The applied force is the
force exerted on the desk by the person.

Gravity Force

(also known as
Weight)

Fgrav

The force of gravity is the
force with which the earth, moon, or other
massively large object attracts another object
towards itself. By definition, this is the weight
of the object. All objects upon earth experience a
force of gravity which is directed "downward"
towards the center of the earth. The force of
gravity on earth is always equal to the weight of
the object as found by the equation:

Fgrav = m *
g

Normal Force

Fnorm

The normal force is the support
force exerted upon an object which is in contact
with another stable object. For example, if a book
is resting upon a surface, then the surface is
exerting an upward force upon the book in order to
support the weight of the book. On occasions, a
normal force is exerted horizontally between two
objects which are in contact with each other. For
instance, if a person leans against a wall, the
wall pushes horizontally on the person.

Friction Force

Ffrict

The friction force is the
force exerted by a surface as an object moves
across it or makes an effort to move across it.
There are at least two types of friction force -
sliding and static friction. Thought it is not
always the cast, the friction force often opposes
the motion of an object. For example, if a book
slides across the surface of a desk, then the desk
exerts a friction force in the opposite direction
of its motion. Friction results from the two
surfaces being pressed together closely, causing
intermolecular attractive forces between molecules
of different surfaces. As such, friction depends
upon the nature of the two surfaces and upon the
degree to which they are pressed together. The
maximum amount of friction force which a surface
can exert upon an object can be calculated using
the formula below:

Air Resistance
Force

Fair

The air resistance is a special
type of frictional force which acts upon objects as
they travel through the air. The force of air
resistance is often observed to oppose the motion
of an object. This force will frequently be
neglected due to its negligible magnitude (and due
to the fact that it is mathematically difficult to
predict its value). It is most noticeable for
objects which travel at high speeds (e.g., a
skydiver or a downhill skier) or for objects with
large surface areas. Air resistance will
be discussed in more detail in Lesson 3.

Tension Force

Ftens

The tension force is the
force which is transmitted through a string, rope,
cable or wire when it is pulled tight by forces
acting from opposite ends. The tension force is
directed along the length of the wire and pulls
equally on the objects on the opposite ends of the
wire.

Spring Force

Fspring

The spring force is the force
exerted by a compressed or stretched spring upon
any object which is attached to it. An object which
compresses or stretches a spring is always acted
upon by a force which restores the object to its
rest or equilibrium position. For most springs
(specifically, for those which are said to obey
"Hooke's Law"), the magnitude of the force is
directly proportional to the amount of stretch or
compression of the spring.

Confusion
of Mass and Weight

A few further comments should be added about the single
force which is a source of much confusion to many students
of physics - the force of gravity. As
mentioned above, the force of gravity acting upon an
object is sometimes referred to as the
weight of the object.
Many students of physics confuse weight with mass. The
mass of an object refers
to the amount of matter that is contained by the object; the
weight of an object is the force of gravity acting upon that
object. Mass is related to how much stuff is there
and weight is related to the pull of the Earth (or any other
planet) upon that stuff. The mass of an object
(measured in kg) will be the same no matter where in the
universe that object is located. Mass is never altered by
location, the pull of gravity, speed or even the existence
of other forces. For example, a 2-kg object will have a mass
of 2 kg whether it is located on Earth, the moon, or
Jupiter; its mass will be 2 kg whether it is moving or not
(at least for purposes of our study); and its mass will be 2
kg whether it is being pushed upon or not.

On the other hand, the weight of an object
(measured in Newtons) will vary according to where in the
universe the object is. Weight depends upon which planet is
exerting the force and the distance the object is from the
planet. Weight, being equivalent to the force of gravity, is
dependent upon the value of
g. On earth's surface
g is 9.8 m/s2
(often approximated as 10 m/s2). On the moon's
surface, g is 1.7
m/s2. Go to another planet, and there will be
another g value.
Furthermore, the g value is inversely proportional to the
distance from the center of the planet. So if we were to
measure g at a distance
of 400 km above the earth's surface, then we would find the
g value to be less than
9.8 m/s2. (The nature of the force of gravity
will be discussed in more detail in a
later unit of The Physics Classroom.) Always be cautious
of the distinction between mass and weight. It is the source
of much confusion for many students of physics.

Sliding
versus Static Friction

As mentioned above, the friction
force is the force exerted by a surface as an object moves
across it or makes an effort to move across it. For the
purpose of our study of physics at The Physics Classroom,
there are two types of friction force - static friction and
sliding friction. Sliding
friction results when an object slides across a
surface. As an example, consider pushing a box across a
floor. The floor surface offers resistance to the movement
of the box. We often say that the floor exerts a friction
force upon the box. This is an example of a sliding friction
force since it results from the sliding motion of the box.
If a car slams on its brakes and skids to a stop (without
antilock brakes), there is a sliding friction force exerted
upon the car tires by the roadway surface. This friction
force is also a sliding friction force because the car is
sliding across the road surface. Sliding friction forces can
be calculated from knowledge of the coefficient of friction
and the normal force exerted upon the object by the surface
it is sliding across. The formula is:

Sliding Ffrict =
 Fnorm

The symbol
represents the coefficient of
sliding friction between the two surfaces. The
coefficient value is dependent primarily upon the nature of
the surfaces which are in contact with each other. For most
surface combinations, the friction coefficients show little
dependence upon other variables such as area of contact,
temperature, etc. Values of
have been experimentally determined for a variety of surface
combinations and are often tabulated in technical manuals
and handbooks. The values of
provide a measure of the relative amount of adhesion or
attraction of the two surfaces for each other. The more that
surface molecules tend to adhere to each other, the greater
the coefficient values and the greater the friction
force.

Friction forces can also exist when the two surfaces are
not sliding across each other. Such friction forces are
referred to as static friction.
Static friction results
when the surfaces of two objects are at rest relative to one
another and a force exists on one of the objects to set it
into motion relative to the other object. Suppose you were
to push with 5-Newtons of force on a large box to move it
across the floor. The box might remain in place. A static
friction force exists between the surfaces of the floor and
the box to prevent the box from being set into motion. The
static friction force balances the force which you exert on
the box such that the stationary box remains at rest. When
exerting 5 Newtons of applied force on the box, the static
friction force has a magnitude of 5 Newtons. Suppose that
you were to push with 25 Newtons of force on the large box
and the box were to still remain in place. Static friction
now has a magnitude of 25 Newtons. Then suppose that you
were to increase the force to 26 Newtons and the box finally
budged from its resting position and was set into
motion across the floor. The box-floor surfaces were able to
provide up to 25 Newtons of static friction force to match
your applied force. Yet the two surfaces were not able to
provide 26 Newtons of static friction force. The amount of
static friction resulting from the adhesion of any two
surfaces has an upper limit. In this case, the static
friction force spans the range from 0 Newtons (if there is
no force upon the box) to 25 Newtons (if you push on the box
with 2 5 Newtons of force). This relationship is often
expressed as follows:

Sliding Ffrict
 Fnorm

The symbol
represents the coefficient of
static friction between the two surfaces. Like
the coefficient of sliding friction, this coefficient is
dependent upon the types of surfaces which are attempting to
move across each other. In general, values of static
friction coefficients are greater than the values of sliding
friction coefficients for the same two surfaces. Thus, it
typically takes more force to budge an object into
motion than it does to maintain the motion once it has been
started.

The meaning of each of these forces listed
in the table above will have to be thoroughly understood to
be successful during this unit. Ultimately, you must be able
to read a verbal description of a physical situation and
know enough about these forces to recognize their presence
(or absence) and to construct a free-body diagram which
illustrates their relative magnitude and direction.

Check
Your Understanding

1. Complete the following table showing the relationship
between mass and weight.

Object

Mass
(kg)

Weight
(N)

Melon

1 kg

Apple

0.98 N

Pat Eatladee

25 kg

Fred

980 N

2. Different masses are hung on a spring scale calibrated
in Newtons.

The force exerted by gravity on 1 kg = 9.8
N.

The force exerted by gravity on 5 kg = ______ N.

The force exerted by gravity on _______ kg = 98
N.

The force exerted by gravity on 70 kg = ________
N.

3. When a person diets, is their goal to lose mass or to
lose weight? Explain.